WASHINGTON – NATO admitted yesterday that a U.S. F-16 accidentally bombed a refugee convoy in Kosovo – but President Clinton accused the Serbs of using civilians as “human shields.”

“You cannot have this kind of conflict withoutsome errors like this occurring,” Clinton said.

“This is not a business of perfection,” he said in SanFrancisco before heading to Detroit and Boston to raise $1.2 million for the Democrats.

Clinton said NATO was “bending over backward” to avoid hitting civilians, and accused Yugoslav President Slobodan Milosevic of using ethnic Albanians as “human shields.”

He stopped short of saying the villagers hit by the F-16 were used that way.

“If [Milosevic] doesn’t want this to happen, he ought to get out of Kosovo,” Clinton contended.

He also hinted that NATO seeks Milosevic’s ouster, saying the region, in the future, “will never be safe with a belligerent tyranny in their midst.”

NATO and U.S. officials initially suggested the Serbs had killed the dozens of civilians in the convoy, but yesterday NATO conceded that an F-16 pilot “dropped his bomb in good faith” – mistakenly believing he was striking Serbian military trucks.

On Capitol Hill, Clinton’s top military aides signaled that the war could stretch into the summer, and tried to brace Congress and the public for the possibility of U.S. casualties as the campaign continued – but denied there were plans to send in ground troops.

“I think the prospect for casualties remains very real and high,” said Joint Chiefs of Staff Chairman Henry Shelton.

Defense Secretary William Cohen agreed that airstrikes could last for months. “This is not going to be quick or easy or neat,” he said.

Cohen also defended the F-16 pilot, saying he was under heavy fire, was flying more than 400 mph and had to make a “split-second decision.”

But the pilot, in a taped interview played by NATO, made no mention of anti-aircraft fire and said he spent 25 minutes scouting the area before dropping his bomb.

He said he was flying high, at 15,000 feet, was hampered at times by clouds, and relied on his “eyeballs” to spot what he described as three dark-green, uniformly shaped vehicles typical of those used to carry Serbian troops.

The Pentagon initially said there was a “possibility” that Serbian forces turned on the civilians after an allied bomb struck their vehicle, and also noted that refugees fleeing over the Albania border were reporting air attacks on them by Serbian helicopters and planes.

A Pentagon spokesman said only that Wednesday’s erroneous explanation was based on “initial” reports that turned out to be baseless.

In other developments:

*Top Republicans and Democrats accused the White House of underestimating the cost of the war. The White House will seek $4 billion to pay for it, but Sen. Bob Kerrey (D-Neb.) said the real price tag could be closer to $8 billion.

House GOP Leader Dick Armey (R-Texas) claimed U.S. forces were being stretched “dangerously thin” elsewhere to pad the Balkans effort.

*More members of Congress turned up the heat on Clinton to consider using ground troops, and demanded he start preparing the GIs now. Among those firing off the letter to the White House were New Yorkers Peter King (R-L.I.), Eliot Engel (D-Bronx), Gary Ackerman (D-Queens), Jerrold Nadler (D-Manhattan) and Major Owens (D-Brooklyn).

But freshman New York Sen. Charles Schumer said he’s opposed to ground troops: “It seems like the wrong place at the wrong time,” he said.

*The airstrikes continued into the 24th day, with explosions heard overnight at Novi Sad, the Belgrade suburb of Pancevo and between the Montenegro cities of Bar and Ulcinj.